This invention relates generally to control systems for electrically propelling traction vehicles such as diesel-electric or straight electric locomotives, and it relates more particularly to improved means for controlling such a vehicle in a manner that avoids or minimizes undesirable slippage of the vehicle wheels when "motoring" (propulsion mode of operation) or "braking" (dynamic retardation mode of operation).
Modern locomotives and other large self-propelled traction vehicles commonly have, per vehicle, at least four axle-wheel sets (each set comprising a pair of wheels affixed to opposite ends of a rotatably mounted axle), with each axle-wheel set being connected via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In the motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power (e.g., an engine-driven traction alternator) and apply torque to the vehicle wheels which exert tangential force or tractive effort on the surface on which the vehicle is traveling (e.g., the parallel steel rails of a railroad track), thereby propelling the vehicle in a desired direction along the right of way. Alternatively, in an electrical braking mode of operation, the motors serve as axle-driven electrical generators; torque is applied to their shafts by their respectively associated axle-wheel sets which then exert braking effort on the surface, thereby retarding or slowing the vehicle's progress. In either case, good adhesion between each wheel and the surface is required for efficient operation of the vehicle.
It is well known that maximum tractive or braking effort is obtained if each powered wheel of the vehicle is rotating at such an angular velocity that its actual peripheral speed is slightly higher (motoring) or slightly lower (braking) than the true vehicle speed (i.e., the linear speed at which the vehicle is traveling, usually referred to as "ground speed" or "track speed"). The difference between wheel speed and track speed is referred to as "slip speed." There is a relatively low limit value of slip speed at which peak tractive or braking effort is realized. This value, commonly known as maximum "creep speed," is a variable that depends on track speed rail conditions. So long as the maximum creep speed is not exceeded, slip speed is normal and the vehicle will operate in a stable microslip or creep mode.
As a practical matter there are times when, due to a deterioration of rail conditions or to an untoward increase in torque, wheel-to-rail adhesion tends to be reduced or lost and some or all of the vehicle wheels will slip excessively, i.e., the actual slip speed will be greater than the maximum creep speed. Such a wheelslip condition, which is characterized in the motoring mode by one or more spinning axle-wheel sets and in the braking mode by one or more sliding or skidding axle-wheel sets, can cause accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of tractive (or braking) effort.
Many different systems are disclosed in the relevant prior art for automatically detecting and recovering from undesirable wheelslip conditions, or for preventing such conditions in the first place. See for example U.S. Pat. Nos. 3,437,896 and 3,728,596. Virtually all wheelslip control or correction systems temporarily reduce or "derate" the traction power in order to cure or to avoid a wheelslip condition. During the period of time that such power reduction is in effect, the productivity of the vehicle is undesirably reduced, and for this reason both the amount of deration and its duration should be minimized.
In normal motoring operation, the propulsion system of a diesel-electric locomotive is so controlled as to establish a balanced steady-state condition wherein the engine-driven alternator produces, for each discrete position of a throttle handle, a substantially constant, optimum amount of electrical power for the traction motors. In practice suitable means are provided for overriding normal operation of the propulsion controls and reducing engine load in response to certain abnormal conditions, such as loss of wheel adhesion or a load exceeding the power capability of the engine at whatever engine speed the throttle is commanding. This response, generally referred to as deration, reduces traction power, thereby helping the locomotive recover from such temporary conditions and/or preventing serious damage to the engine.
In addition, the propulsion control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of this voltage and the magnitude of load current from respectively exceeding predetermined safe maximum levels or limits. Current limit is effective when the locomotive is accelerating from rest. At low locomotive speeds, the traction motor armatures are rotating slowly, so their back EMF is low. A low alternator voltage can now produce maximum motor current which in turn produces the high tractive effort required for acceleration. On the other hand, the alternator voltage magnitude must be held constant at its maximum level whenever locomotive speed is high. At high speeds the traction motor armatures are rotating rapidly and have a high back EMF, and the alternator voltage must then be high to produce the required load current.
Some of the prior art wheelslip control systems attempt to curtail undesirable wheel slippage by monitoring slip speed and overriding the normal propulsion controls so as to reduce traction power if and when the slip speed exceeds a permissible limit which depends on the magnitude of motor current (and hence total motor torque). See U.S. Pat. No. 4,463,289 and British Pat. No. 1,246,053. Such creep regulators are advantageous because they minimize the duration and severity of wheelslip conditions, thereby enhancing the useful life of the vehicle wheels and the productivity of the vehicle.
Most prior art wheelslip controls use the difference between the rotational speed of the fastest axle-wheel set and the rotational speed of the slowest axle-wheel set to indicate slip speed. This difference is a true measure of slip speed so long as at least one axle-wheel set is not slipping and at least one set is. But if all sets have the same speed, the difference speed will be zero and therefore cannot indicate whether the vehicle is operating properly, with all of its powered wheels rotating at maximum creep speed, or is experiencing a "synchronous" wheelslip condition, with the slip speeds of all axle-wheel sets simultaneously exceeding the maximum creep speed. Various prior art solutions to the problem of detecting synchronous slips are disclosed in U.S. Pat. Nos. 3,210,630; 4,075,538; and 4,588,932, respectively.
Many prior art wheelslip control systems are operative in response to rapid changes of wheel speed. A wheelslip condition is indicated if the rate at which the rotational speed of an axle-wheel set is changing increases above a predetermined threshold rate that somewhat exceeds the maximum acceleration (in the motoring mode) or deceleration (in the braking mode) obtained during normal speed changes of the vehicle. See, for example, U.S. Pat. Nos. 3,541,406; 3,482,887; and 4,035,698.
The creep or slip of each powered wheel of the vehicle can be accurately detected by comparing the speed of the axle-wheel set with the actual ground speed of the vehicle. But this requires equipping the vehicle with suitable means for detecting ground speed such as a radar unit (which is relatively expensive and requires continuous recalibration to avoid errors due to vibration or wheel wear) or an extra, unpowered wheel or roller (which is cumbersome and unreliable).
Another known technique for avoiding wheelslip during motoring is to control or regulate the voltage applied to the traction motors so as to hold maximum creep speed. In U.S. Pat. No. 3,982,164, the maximum voltage limit of an open-loop propulsion control system is set a programmed amount in excess of the synthesized voltage of a non-slipping motor, which programmed amount represents the desired maximum percent slip and is automatically adjusted to maximize the combined motor currents and hence total motor torque. Alternatively, as is shown and described in Canadian Pat. No. 950,559, a closed-loop voltage regulator will tend to arrest incipient wheelslips, and the control loop can also include means for reducing the voltage set point (and hence motor speed) in immediate response to the onset of a wheelslip condition as indicated by excessive slip speed (see page 29 of the referenced Canadian patent).